This invention relates to an electromagnetic drive system comprising an electromotor of which a first motor section is disposed movably relative to a second motor section, the second motor section comprising a first coil and the first motor section comprising a magnetic circuit for generating a magnetic field in the first coil, one of the motor sections comprising a short-circuit winding having an electromagnetic coupling to at least two coils of the other motor section, the electromagnetic coupling between the short-circuit winding and at least one of the latter coils being dependent on the position of the first motor section relative to the second motor section.
A drive system of this type is known from U.S. Pat. No. 4,616,153. In the prior-art drive system the motor sections are constituted by two coaxial cylindrical bodies which are disposed slidably relative to each other. The internal cylindrical body constitutes the first motor section consisting of a tube around which an excitation coil and the short-circuit winding is disposed. The external cylindrical body comprises the magnetic circuit which generates a radially directed magnetic field in the excitation coil. The second motor section further includes two detection coils disposed coaxially relative to the short-circuit winding and having a position-dependent electromagnetic coupling to the short-circuit winding.
By injecting a high-frequency detection signal into the detection coils, the position of the first motor section relative to the second motor section can be derived from the signals across the detection coils. In this manner a motor with a built-in position detector is realised. A disadvantage of the prior art drive system, however, is that the components necessary for position detection take up a relatively large amount of space so that the drive system is less suitable for applications in which it is desired that the motor have very small dimensions, such as, for example, miniatufised portable recording systems such as are used, for example, with an electronic still picture.